1. Field of the Invention
The present invention relates to an elastomer molded by reaction injection molding, and more particularly, to an elastomer molded by a reaction injection molding of polyurethane resins, polyurea resins, polyurethane/urea resins and/or polyurea/amide resins using a composition containing a polyester condensate as an internal release agent, and a process for production thereof.
2. Description of the Related Art
Methods for producing molding articles of polyurethane resins, polyurea resins, and other synthetic resins by means of reaction injection molding (hereinafter referred to as "RIM") are known.
RIM is an injection molding method comprising sufficiently mixing an active hydrogen component and a polyisocyanate component and then rapidly curing and molding the resulting mixture in a mold, and it is a tape of so-called "one-shot" molding methods.
The "active hydrogen component" is a component having a functional group containing an active hydrogen capable of reacting with an isocyanate group. The functional groups containing an active hydrogen are hydroxyl group, amino group, imino group (including enamine in the relation of tautomerism with imino group) and the like. The reaction of a hydroxyl group with an isocyanate group is a urethane condensation and the resulting resin is a polyurethane resin. In the following, when an amino group reacts with an isocyanate group, the resulting resin is called "polyurea resin"; when both hydroxyl group and amino group react with isocyanate groups, the resulting resin is called "polyurethane/urea resin"; and when an imino group reacts with an isocyanate group, the resulting resin is called "polyurea/amide resin".
In a RIM technique, elastomers such as polyurethane, polyurea and the like mean non-foamed elastomers, micro-cellular elastomers and the like, and the density is about 0.8 g/cm.sup.3 or more. When no filler is contained, the upper limit of density is 1.2 g/cm.sup.3. The elastic elastomers are used for external parts of a car and the like.
A feature of RIM resides in the short molding time between injection of the material and releasing the molded article, but the total time for the molding cycle is not yet sufficiently short at present. A big problem in shortening the time for the molding cycle is that a long time is required for cleaning the mold and applying a release agent thereto. In order to solve this problem, it is important from a standpoint of productivity that molded articles can be easily released from molds. For attaining this purpose, various internal release agents have been developed.
It is desired that internal release agents exhibit excellent solvent stability in the raw materials and excellent releasability from molds, and neither disturb the reactivity upon heat curing nor cause fish eyes, defective adhesion and the like upon coating the resulting molded article.
Japanese Patent Application Laid-open No. Sho 61 -91216 discloses an internal release agent for producing a polyurethane elastomer having an average density of 0.8-1.4 g/cm.sup.3. The internal release agent is an ester group-containing condensate produced from 3-15 moles of ricinoleic acid and 1 mole of monohydric or polyhydric alcohol which has a molecular weight of 900-4500, an acid value of 5 or less and an OH value of 12.5-125.
The ester group-containing condensate derived from ricinoleic acid exhibits solvent stability in the material and gives a good coating property.
However, the releasing property is not sufficient and further reducing the acid value to 2 or less is very difficult in the synthesis, and therefore, when a large amount of the internal release agent is added, the polyaddition reaction of polyurethane is disturbed.
Further, when a molded article is released from a mold, the stiffness of the polyurethane molded article is so poor that deformation is liable to occur, and therefore, the handling in production becomes complicated and the productivity is markedly deteriorated. In addition, with respect to physical properties, the heat resistance is lowered.
The present inventors intensively carried out research to develop elastomers exhibiting excellent continuous releasability and coating property. As a result, they have found that ester group-containing condensates derived from 12-hydroxystearic acid and alkanediol do not lower stiffness and heat resistance as to the physical properties of molded articles and the resulting invention has been already filed for a patent application as Japanese Patent Application No. Hei 2 - 214311.
The present inventors have found that an elastomer excellent in continuous releasability and coating property can be produced by using an internal release agent produced by reacting a hydroxy fatty acid ester or a mixture of said ester and a polyether polyol with a diisocyanate to prepare a prepolymer and substituting the ends of the molecule of the prepolymer with amino group (hereinafter called "end-amine-modifying technique"), and this invention has been filed as Japanese Patent Application No. Hei 2 -246282.
The present inventors have continued the research and found that when the condensation of an ester of a saturated fatty acid having a hydroxyl group with an alkanediol or diol having an ether group is carried out by transesterification, there is obtained an ester group-containing condensate having a very low acid value and a remarkably improved releasing property as compared with a conventional ester group-containing condensate produced by dehydration of fatty acids and alkanediols (hereinafter called "transesterification technique").
The present inventors have further found that when this transesterification technique is applied to the above-mentioned end-amine-modifying technique, the characteristics of the internal release agent are further improved.
As a result, the present invention has been totally accomplished.